This invention relates to three-phase and three-leg core structures of core-type transformers, and more particularly to a three-phase and three leg core structure of a core-type transformer having a yoke of a no-circular cross-sectional shape, wherein each of the main legs and the yoke are substantially equal to each other in cross-sectional area (including cases where the cross-sectional area of the yoke is greater by up to about 15% than the cross-sectional area of the main legs). In particular, the invention deals with the construction of a miter joint formed by each of steel plates constituting the main legs and each of steel plates constituting the yoke.
In cores of core-type transformers described above, a main leg 1 is stepped in cross-sectional shape, as shown in FIG. 1(a), in a manner such that the leg 1 may conform to the shape of the inner surface of a circular coil to maximize the internal space of the circular coil occupied by the leg. Meanwhile the yoke 2 is available in a variety of cross-sectional shapes including a circular shape and non-circular shapes, such as square, semi-elliptic and elliptic shapes, as shown in FIG. 1(b) to FIG. 1(d). While the yoke of a circular cross-sectional shape conforms to the shape of the inner surface of the circular coil as does the leg 1, the yokes of non-circular cross-sectional shapes have been fabricated for the purpose of reducing the height of the yoke to overcome transportation difficulty and other restrictions.
It is publicly known that in a three-phase and three-leg core of a core-type transformer wherein the yoke 2 has a circular cross-sectional shape as does the main leg 1, steel plates 1a to 1c (See FIG. 2) constituting the main leg 1 have a width l.sub.1 which varies depending on the position of the steel plates 1a to 1c in a stack of steel plates forming a core. As the width l.sub.1 of the steel plates 1a to 1c varies, the width l.sub.2 of steel plates 2a to 2d constituting the yoke 2 also varies, so that the steel plates 1a to 1c of the main leg 1 and the steel plates 2a to 2d of the yoke 2 have the same width at all times. As shown in FIG. 2, the angle .theta. formed by a joint 4 between the main leg 1 and the yoke 2 and the axis 3 of the main leg 1 (hereinafter referred to as the joint angle) is 45.degree. at any position in the stack of steel plates forming the core as is customary with anisotropic silicon steel plates.
However, if the steel plates are joined in the manner shown in FIG. 2 in a three-phase and three-leg core of a core-type transformer wherein the yoke is non-circular in cross-sectional shape, the following problems will arise.
Let us proceed with the case in which the yoke 2 is square in cross-sectional shape. In this case, as shown in FIG. 3, the width l.sub.2 of the steel plates constituting the yoke 2 and the widths l.sub.11 to l.sub.18 of the steel plates constituting the main leg 1 are substantially related to one another as follows: EQU l.sub.11 &lt;l.sub.12 &lt;l.sub.13 &lt;l.sub.14 &lt;l.sub.2 &lt;l.sub.15 &lt;l.sub.16 &lt;l.sub.17 &lt;l.sub.18
The result of this is that, if one attempts at fabricating a three-phase and three-leg core having a yoke of a non-circular cross-sectional shape by joining the steel plates at the customary joint angle of 45.degree., the core will be constructed as shown in FIG. 4(a) when the width l.sub.2 of the steel plates constituting the yoke 2 is smaller than the width l.sub.1 of the steel plates constituting the main leg 1 or l.sub.2 &lt;l.sub.1 and will be as shown in FIG. 4(b) when l.sub.2 &gt;l.sub.1.
Because of this, the steel plates constituting the yoke 2 and the main leg 1 include a large number of steel plates which require end cutting at one or both ends as shown in FIG. 5(a) and FIG. 5(b). This has disadvantages in that a cutting operation is troublesome and in addition the yield of the products is low.
Also, an anisotropic silicon steel plate has a magnetic permeability which, as shown in FIG. 6, becomes markedly low in a portion of the steel plate which deviates from the direction in which rolling has been performed. In actual practice, a zone of the steel plate which is more than 50.degree. away from the rolling direction has a magnetic permeability which is below 1/100 the magnetic permeability of the zone of the steel plate disposed in the rolling direction. Calculation of a magnetic field conducted recently by taking into consideration the anisotropicity of magnetic permeability clearly shows that the magnetic flux in the interior of the core using anisotropic silicon steel plates flows in the rolling direction in which there is a high magnetic permeability. Thus, it will be seen that when a core is constructed as shown in FIG. 4(a) the flow of the magnetic flux will be non-uniform at the joints of the steel plates as shown in FIG. 7(a) and FIG. 7(b).
More specifically, at a joint formed by each of the steel plates 1a constituting the main leg 1 (hereinafter referred to as the main leg steel plate) and each of the steel plates 2a constituting the yoke 2 (hereinafter referred to as the yoke steel plate), the magnetic flux is concentrated in an inner corner portion of the main leg steel plate 1a, with almost no magnetic flux flowing in an outer marginal portion thereof. Thus the main leg steel plates of a large width are not utilized effectively. Also, concentration of the magnetic flux is noted at joints formed by the yoke steel plates 2a and 2b and the main leg steel plate 1b, as shown in FIG. 7. This has disadvantages in that a local loss occurs and core loss is increased.
On the other hand, proposals have been made to change the joint angle in a core structure. One example of such proposals involves the use of a single-phase and two-leg core structure shown in FIG. 9. However, in this core structure, the joint formed by the main leg steel plate 1a and the yoke steel plate 2a is in the form of a straight line ST starting at an inner corner point S and extending to an outer corner point T at an angle .theta..sub.1, and the joint of the adjacent steel plate layer is in the form of a straight line SU starting at the inner corner point S and extending to a point U at an angle of .theta..sub.2. Stated differently, the width of the overlapping portions of the adjacent steel plate layers increases in going outwardly from the inner corner point.
Because of this arrangement, the steel plates have a reduced lap dimension (the length of the overlapping portions of the adjacent steel plate layers) on the inner side, and the steel plates will be joined in a butt joint (a joint wherein the lines of the joints of the adjacent steel plate layers are disposed in substantially the same vertical plane with respect to the layers) on a considerable scale, if there is error in the operations of stacking and inserting the steel plates to fabricate a core. This has disadvantages in that core loss and the value of an exciting current are markedly increased depending on how the operations are performed and the cores produced are not stable in quality. Conversely, if the lap dimension of the adjacent steel plate layers is increased, there will be the disadvantages of the amount of end cut steel material increasing and of the yield of the steel plates reducing.
Moreover, if this joint structure is applied to a three-phase and three-leg core structure, the butt joint portion on the inner side will cause the magnetic flux to be concentrated in the outer side of the core because of high magnetic reluctance on the inner side of the core, thereby increasing core loss.